The present invention relates to a residual stress improving method for members in a reactor pressure vessel.
It is generally known that metallic materials such as austenite stainless steel used for members in a nuclear plant pressure vessel cause intergranular stress corrosion cracking (hereinafter abbreviated as IGSCC) in welded portions of the members or thereabout when disposed in water at high temperature within the reactor.
IGSCC is caused under conditions where three factors of materials, i.e., sensitization, tensile stress, and corrosive environment, are combined with each other.
Sensitization of materials is developed owing to that the welding heat, for example, precipitates a chromium carbide in the grain boundary and, hence, a Cr depleted layer which is more sensitive to corrosion is formed near the grain boundary (this process being called sensitization).
Tensile stress is produced in combination of stress caused by an external force and tensile residual stress caused in a material surface during processes of welding and machining.
Corrosive environment is developed with the presence of high temperature water containing dissolved oxygen.
IGSCC can be prevented by eliminating any one of those three factors.
Examples of known prior art for improving residual stress in a material surface, that is the cause of producing tensile stress, aiming to prevent IGSCC are as follows.
(1) JP, A, 62-63614
In this known prior art, a peening apparatus using high pressure water is inserted to pipes of a heat exchanger or the like, and a high pressure water jet is ejected from a rotating nozzle to effect peening for transformation from tensile residual stress of the pipe into compressive residual stress.
(2) JP, A, 5-78738
In this known prior art, a water jet is ejected from a nozzle toward surfaces of members in a reactor plant pressure vessel in water for impinging a water jet stream accompanied with cavitation bubbles against the member surface. As a result, the member surface is subjected to peening for effective transformation from tensile residual stress in the member surface into compressive residual stress.
(3) JP, A, 53-21021
In this known prior art, an induction heating coil is disposed around a pipe of austenite stainless steel to heat the pipe, and cooling water is then ejected into the pipe to rapidly cool an inner surface of the pipe for transformation from tensile residual stress in the pipe inner surface into compressive residual stress.
(4) JP, A, 60-258409
In this known prior art, small balls or the like at low temperature are blown toward a surface of a metallic member at a high speed to effect penning for transformation from tensile residual stress in the metallic member surface into compressive residual stress.
(5) JP, A, 4-362124
This known prior art discloses that high pressure water is ejected toward a surface of a metallic member from a nozzle in water or aquatic environment created in the atmosphere for impinging the ejected water accompanied with cavitation bubbles against the metallic member surface. With the energy produced when the cavitation bubbles collapse, the metallic member surface is peened to cause compressive residual stress in the metallic member surface. This known prior art also discloses that a horn-shaped nozzle adapted to easily produce the cavitation bubbles is used as the nozzle in the water.
(6) JP, A, 5-195052
This known prior art, corresponding to U.S. Pat. No. 5,305,361, discloses that a nozzle for ejecting high pressure water toward a surface of a metallic member in water is vibrated at high frequency to induce cavitation bubbles near a vibrating nozzle surface, and the cavitation bubbles are impinged along with a high pressure water stream ejected from the nozzle against the metallic member surface. With the energy produced when the cavitation bubbles collapse, the metallic member surface is peened to cause compressive residual stress in the metallic member surface. This known prior art also discloses that a horn-shaped nozzle adapted to easily produce the cavitation bubbles is used as the nozzle in the water.
However, the above-mentioned prior arts have problems as follows.
In the known prior art (1), a water jet ejected from a nozzle is impinged against a material surface in the atmosphere to effect peening with impinging pressure resulted by the ejected water jet. For nuclear plant materials which must be treated in water, the water jet would be diffused and its flow speed would be lowered due to resistance of the surrounding water before reaching the metal surface. Therefore, the peening effect cannot be achieved with a satisfactory result. Thus, the known prior art (1) has a difficulty in application to the nuclear plant materials.
To achieve satisfactory peening in the known prior art (2), a flow speed of the water ejected from a nozzle must be so very high that the water jet can produce cavitation bubbles in a sufficient amount. Accordingly, the known prior art (2) is disadvantageous in needing a pump of great horse power and, hence, entailing large-sized and intricate arrangements
The known prior art (3) is applied to a pipe by winding an induction heating coil around the pipe to be treated. Therefore, the known prior art (3) cannot be applied to a large-sized structure and, hence, has a difficulty in practical use for improving residual stress of large-sized members in a reactor pressure vessel.
In the known prior art (4), blowing small balls or the like in water is difficult because of resistance of the surrounding water as with the above known prior art (1). Also, it is very troublesome to collect the blown small balls or the like when the treating operation is carried out in the nuclear plant. Thus, the known prior art (4) has a difficulty in application to the nuclear plant materials.
To achieve satisfactory peening in the known prior art (5), a flow speed of the ejected water must be so very high, as with the above known prior art (2), that the water jet can produce cavitation bubbles in a sufficient amount. Accordingly, the known prior, art (5) is also disadvantageous in needing a pump of great horse power and, hence, entailing large-sized and intricate arrangements.
The known prior art (6) requires complicated arrangements because a means for vibrating the nozzle at high frequency is needed to produce cavitation bubbles in a sufficient amount.
Further, in attempting to improve residual tensile stress of members in a reactor pressure vessel, any of the foregoing prior arts raises a problem that the size and complexity of equipment used for the improvement, such as a pump and a heating/cooling apparatus, is increased.
Accordingly, there has been a demand for improving residual tensile stress of members in a reactor pressure vessel while preventing the necessity of large-sized and intricate equipment as far as possible.
An object of the present invention is, therefore, to provide a residual tensile stress improving method for members in a reactor pressure vessel which can improve residual tensile stress of the members in the reactor pressure vessel with simple arrangements.